CN101960809A

CN101960809A - DC compensation

Info

Publication number: CN101960809A
Application number: CN200980106781XA
Authority: CN
Inventors: A·B·修斯
Original assignee: Nokia Oyj
Current assignee: Nokia Technologies Oy
Priority date: 2008-02-28
Filing date: 2009-02-23
Publication date: 2011-01-26
Anticipated expiration: 2029-02-23
Also published as: WO2009106494A1; KR101268753B1; EP2248315A1; GB0803710D0; US8396167B2; KR20100132010A; EP2248315B1; US20110013728A1; CN101960809B

Abstract

An apparatus comprising an analog fiiter, an analog to digital converter coupled to said analog filter; and a digital filter coupled to said analog to digital converter; wherein the apparatus is configured such that distortion introduced into a filtered signal by said analog filter is substantially compensated by said digital filter.

Description

The DC compensation

Technical field

Present invention relates in general to communication, and more specifically, relate to the DC compensation in the RF receiver, and relate to especially and not exclusively based on the DC in the system of orthogonal frequency division modulated (OFDM) and compensating.

Background technology

In orthogonal frequency division modulated (OFDM), data are being transmitted on a plurality of frequencies within the duration T of symbol time.The character of OFDM is, the frequency interval 1/T of individual carrier wave, and wherein T is the duration of data symbol.This character allows to utilize the ideal frequency selectivity between the individual carrier wave to make the OFDM receiver.In other words, the carrier wave in the OFDM waveform as follows at interval: they lose each other at receiver side effectively, and also, carrier wave is orthogonal, thereby make and do not exist cross interference not have the loss of signal.

The benefit of OFDM is the spectral efficient (throughput of channel width/MHz) and multipath disturbed and the high antagonism of frequency selective fading.

In order to keep the carrier wave orthogonality in having the multi-path channel that postpones expansion, the OFDM symbol is effectively transmission in the time of slightly being longer than T.At receiver side, be not that detection OFDM symbol is needed this extra time, therefore can be considered to protection period (GP).For exemplary systems, T can be that 67 μ s (Δ f=15kHz) and GP can be 0.5 μ s.

Usually be to utilize around the carrier wave symmetry of channel center frequency to define ofdm system easily, also promptly, for channel center, exist in+/-N/2 * Δ f place carries the subcarrier of data symbol, wherein N is the sum of subcarrier, and Δ f is a subcarrier spacing.In this respect, 0Hz carrier wave or DC carrier wave are exceptions, are difficult to avoid loss on this carrier wave because the transmitter and receiver technology makes.Reason for this reason, the DC carrier wave is sacrificed usually.Consider that from the angle of frequency spectrum the DC carrier wave can be considered to any other subcarrier.If complicated DC level is constant in the duration of symbol time, it will not disturb other data symbols, also, and DC subcarrier and the data quadrature that carries subcarrier.

When the RF receiver of design ofdm system, a selection can be directly to change receiver.Directly changing in the receiver, the direct and base band mixing of RF signal, also, local oscillator frequencies equals channel frequency.Therefore, the DC component of intrinsic generation in the RF frequency mixer will be present in the DC subcarrier, and if the DC level constant at the duration of symbol, it will be harmless for the detection of data symbol in other subcarriers in theory.

Yet when receiving little RF signal, the DC component may be significant with respect to desired signal.Under extreme case, the DC level may surpass expected signal levels and reach great magnitude.Because the electronic equipment particularly dynamic range of A/D converter is limited, therefore is desirably in the A/D conversion usually and in base band, removes before the DC component.

Unfortunately, be difficult to remove the DC component and do not make distorted signals in the mode that influences other detection of subcarrier signal.This is because for a variety of reasons, the DC level changes at the duration of symbol usually.The Change Example that causes the DC level is as being the strong external drag in the frequency acceptance band, and it generates signal in the DC subcarrier.And when the control in automatic gain control (AGC) system gains down when changing in the front end of receiver and/or rear end, the DC level changes usually.

A method that can be used for eliminating the DC component is to be that the arrowband at center stops filter (perhaps high pass filter) with 0Hz.The cut-off frequency scope of this filter usually can be from the fraction of subcarrier spacing up to subcarrier spacing.

The high pass filter (HPF) that use has very narrow cut-off frequency has following shortcoming, and promptly this filter has the impulse response more much longer than symbol time T.Therefore, this will cause the intersymbol interference (ISI) that makes the symbol detection degradation.And the purposes that filter removes DC is not optimum, because the adjustment time of filter is longer, so it will take the duration of a plurality of OFDM symbols before reducing DC.

The HPF that use has higher cut off frequency (for example, equaling n times of subcarrier spacing) has following shortcoming, and promptly it has destroyed the orthogonality of subcarrier and DC carrier wave.As commonly used in the digital receiver, the equalization of estimating based on transmission channel can't remove the distortion that is caused by HPF.Another viewpoint to this problem is, because HPF adjusts, the DC level after the HPF will can be inconstant in whole OFDM symbol time.When DC is non-constant, power spectrum will expand to other subcarriers, and make the modulated symbols distortion in these subcarriers.

The purpose of some execution mode of the present invention is to solve or alleviate some problem in these problems at least.

Summary of the invention

According to a first aspect of the present invention, a kind of device is provided, comprise analog filter; Be coupled to the analog to digital converter of described analog filter; And the digital filter that is coupled to described analog to digital converter, wherein dispose this device, make the distortion of introducing through filtering signal by described analog filter compensate by described digital filter in fact.

According to an embodiment of the invention, one frequency response can come down in described analog filter and the described digital filter the contrary of another in analog filter and the digital filter.Analog filter can comprise high pass filter.High pass filter can have the frequency response of being determined by following formula:

A (s) = \frac{s}{s + \frac{1}{τ}}

Wherein τ is the time constant that is associated with analog filter.

Digital filter can comprise IIR filter.Digital filter can have the frequency response of being determined by following formula:

D (z) = \frac{1 - a \cdot z^{- 1}}{1 - z^{- 1}}

Wherein a is a filter coefficient.This filter coefficient can be determined by following formula:

a = e^{\frac{- 1}{τ \cdot fs}}

Wherein fs is a sample frequency.

Digital filter can comprise filter memory.Described filter memory is configured to regularly be reset.According to an execution mode, filter memory can be configured to be reset during the protection period before the reception data symbol.

This device can also comprise following at least one: be coupling in the low pass filter between described analog filter and the described analog to digital converter; Be coupling in the baseband amplifier between described low pass filter and the described analog to digital converter; Be coupling in the channel model between described analog to digital converter and the described digital filter; Perhaps be coupled to the frequency mixer of the input of analog filter, wherein said mixer configuration is used for the signal and the mixing frequency that receive are carried out mixing.

This device can also dispose and be used for received RF signal.Radiofrequency signal can comprise the signal of orthogonal frequency division modulated.

According to a second aspect of the present invention, a kind of method is provided, comprising: utilize first filter that analog signal is carried out filtering; In analog to digital converter to changing through filtered analog signals; And utilize second filter that filtering is carried out in described sampling, wherein introduce described distortion and compensate by described second filter in fact through filtered analog signals by described first filter.

One frequency response comes down in described first filter and described second filter the contrary of another in first filter and second filter.First filter can comprise high pass filter.High pass filter can have the frequency response of being determined by following formula:

A (s) = \frac{s}{s + \frac{1}{τ}}

Wherein τ is the time constant that is associated with analog filter.

Second filter comprises IIR filter.Second filter can have the frequency response of being determined by following formula:

D (z) = \frac{1 - a \cdot z^{- 1}}{1 - z^{- 1}}

a = e^{\frac{- 1}{τ \cdot fs}}

Wherein fs is a sample frequency.

Preferably, second filter can comprise filter memory.This method can also comprise: the filter memory of regularly resetting.The replacement filter memory can be timed to during the protection period before the reception data symbol and take place.

This method can also comprise: the signal and the mixing frequency that receive are carried out mixing, to generate described analog signal.Preferably, the signal of reception can comprise radiofrequency signal.

Description of drawings

Now, embodiments of the present invention are described with reference to the accompanying drawings for the mode by example only, wherein:

Fig. 1 shows the receiver according to one embodiment of the present invention.

Fig. 2 shows the equalization filter according to one embodiment of the present invention.

Fig. 3 shows according to the analogous diagram of the output of high pass filter of one embodiment of the present invention and equalization filter and paints.

Fig. 4 shows the method according to one embodiment of the present invention.

Embodiment

To embodiments of the present invention be described by the mode of specific example and with particular reference to preferred implementation at this.It will be understood to those of skill in the art that the details that the invention is not restricted to specific implementations given herein.

Some execution mode of the present invention provides a kind of method, is used for removing the DC component of the Analog Baseband of receiver, and minimizes and remove the distortion that the DC component is associated from signal.According to an embodiment of the invention, DC keeps constant in whole OFDM symbol time.The DC component only changes during the protection period between the OFDM symbol.

According to an execution mode, use the first rank mimic high pass filter that baseband signal is carried out filtering.Can select high pass filter (HPF) to have high relatively cut-off frequency, make the HPF rapid adjustment, preferably in the time of one or two OFDM symbol, adjust.As mentioned above, the HPF with relative higher cutoff frequency will make distorted signals, thereby make it destroy the orthogonality of subcarrier and DC carrier wave.

According to some execution mode of the present invention, can in the wireless impulse response in first rank (IIR) filter, in digital baseband, carry out further filtering to signal.Can carefully select the iir filter coefficient,, thereby remove or remove in fact distortion, recover or recover in fact the orthogonality of subcarrier and DC carrier wave thus from signal so that the distortion that distortion that will be caused by IIR and HPF are caused is complementary.

An embodiment of the invention have been shown among Fig. 1.Fig. 1 shows for receiving the receiver chain that the RF signal uses.This receiver chain can be implemented as the portable terminal in the wireless network for example or the part of base station.

The block diagram of Fig. 1 is the simplified block diagram of receiver chain 1; Common this receiver chain will use in quadrature receiver, and it all has I (homophase) branch and Q (quadrature) branch after having two frequency mixers and each frequency mixer.For quadrature receiver, analog filtering and digital filtering be independent the execution in each of two branches.

The receiver chain 1 of Fig. 1 can be divided into three territories.In radio frequency (RF) analog domain, antenna 22 is coupled to the input of amplifier 2, and amplifier 2 for example is low noise amplifier (LNA).The output of amplifier 2 is coupled to frequency mixer 4, and it also receives mixing frequency (not shown).The output of frequency mixer 4 is coupled to the input of high pass filter (HPF) 6, and the output of HPF 6 is coupled to the input of low pass filter (LPF) 8.The output of LPF 8 is coupled to the input of baseband amplifier 10, and the output of amplifier is coupled to the input of analog to digital converter 12.

In the RF numeric field of the receiver chain of Fig. 1, digital sample is the output of ADC 12, and is coupled to the input of channel model 14.Channel model 14 is coupled to equalization filter 16, and it also receives CP-reset signal 20.The output of equalization filter 16 is coupled to the Base-Band Processing 18 in the numeric field then.

Consider the operation of receiver chain 1, radio signal can be received by antenna 22, and transmits so that amplify to amplifier 2.Then in frequency mixer 4, carry out mixing through amplifying signal with the mixing frequency.Usually, the mixing frequency can be selected like this, makes that the received signal through amplifying is down converted in the base band in frequency mixer 4.Then in HPF 6, carry out high-pass filtering through down signals, thereby suppress low frequency component, comprise any DC component through down-conversion signal.Component through high-pass filtering carries out low-pass filtering then in LPF 8.This signal through low-pass filtering amplifies in baseband amplifier 10 then.The gain of baseband amplifier 10 can be set, use the complete scope of analog to digital converter with the input of guaranteeing ADC 12, and realize good precision thus.

Although described embodiments of the present invention in this context of the received signal that is mixed to base band, embodiments of the present invention are equally applicable to the receiver that signal is mixed to intermediate frequency.

Digital sample (it the represents the RF signal) filtering in channel model 14 then of ADC 12 outputs is to isolate interested frequency.Before transmitting with the decoding received signal to Base-Band Processing 18, then can filtering once more in wireless impact response filter 16 through the sampling of filtering.

According to embodiments of the present invention, HPF 6 is first rank mimic high pass filters.HPF removes the DC component in the Analog Baseband.Can select cut-off frequency higher for the frequency interval of OFDM symbol, thereby make HPF rapid adjustment in the time of one or two OFDM symbol.According to an embodiment of the invention, the cut-off frequency of HPF can be chosen between a thirtieth and OFDM mark space one times of OFDM symbol frequency.

In digital baseband, after analog-to-digital conversion, equalization filter 16 (comprising the specific first rank iir filter that characteristic general and HPF 6 is complementary) can remove the distortion by HPF 6 introducings.

Remove the distortion of introducing by HPF 6 for equalization filter 16, equalization filter can design in such a way, be that the combination frequency response of HPF and equalization filter has the homogeneous gain and (also is from DC to interested base band, flat response), this can be determined by the sample rate in the digital filter.

According to an embodiment of the invention, filter can use single delay element (or claiming filter memory).The value of sampling in the memory will be represented the instantaneous DC level in the digital filter input.Filter memory can regularly be forced particular value.According to an embodiment of the invention, filter memory can be forced zero.In one embodiment, the filter memory of can during each protection period of received signal, resetting.

An example embodiment of equalization filter 16 has been shown among Fig. 2.The filter of Fig. 2 comprises amplifier 30, first adder 32, delay element 34 and second adder 36.Apply the sampling of equalization filter to first input of the input of amplifier 30 and second adder 36.The gain of amplifier 30 can equal-a, and wherein a is a filter coefficient.Sampling through amplifying is coupled to first input of first adder 32.The output of first adder 32 is coupled to delay element 34, and the output of delay element 34 is coupled to second input of second adder 36.Second adder 36 is with the output and the input sample addition of delay element 34, and the output result.The output of second adder 36 is coupled to second input of first adder 32, and with sampled value addition through amplifying, to form the output of first adder 32.The output of second adder 36 also forms the output of equalization filter 16.

Only show two of being equal in the filter that the I branch and the Q branch that are used for to receiver carry out independent filtering.

Thereby make equalization filter be effective as " inverse filter " for the characteristic that makes equalization filter and HPF filter in analog domain is complementary, introduce zero in the z territory of equalization filter, it has eliminated the polarity of HPF in the s territory.Then can be by in numeric field, using the harmful effect that " inverse filter " eliminates HPF.This filter will return to the DC in the numeric field analog gain and upgrade middle constant level, also, and in the middle of the DC in the output of RF frequency mixer jumps.Suppose that the high pass filter in the analog RF territory is the first following rank iir filter of form:

A (s) = \frac{s}{s + \frac{1}{τ}}

Wherein τ is the time constant of HPF, and s is an angular frequency.The 3dB cut-off frequency is:

f_{3 dB} = \frac{1}{2 \cdot π \cdot τ}

Then equalization digital filter can be taked following form:

D (z) = \frac{1 - a \cdot z^{- 1}}{1 - z^{- 1}}

Wherein coefficient a can derive from eliminate A (s) in directly coupling z one pole, zero conversion:

, wherein fs is a sample frequency.

Filter coefficient " a " can depend on the sample rate of using in the numeric field.Using in the system of different sample rates at different mode, " a " coefficient must correspondingly change.

Although the constant DC level in the filter output is harmless to the OFDM symbol decoding, still may expect to remove this DC level, to improve dynamic range through filtering signal.The equalization digital filter of Fig. 2 allows easily to remove filter memory, so that remove the unnecessary DC level in the filter output.According to an embodiment of the invention, filter memory can be removed before each OFDM symbol, also, removed in the protection period (GP) before each OFDM symbol.Remove filter memory in this way and can allow to remove unnecessary DC level, guarantee that simultaneously the DC level keeps constant duration of each symbol.

Fig. 4 shows the method according to one embodiment of the present invention.In the method for Fig. 4, in step 100, to analog signal sampling, this introduces signal with some distortion in first filter.In step 102, in digital to analog converter, change then through the signal of filtering.In step 104, in second filter, digital signal is sampled this compensation or compensated the distortion of in first filter, introducing in fact.

Figure shown in Fig. 3 has shown the exemplary simulations of the operation of equalization filter 16 when application has the input of high DC component.Only show among I of branch or the Q.Input signal to equalization filter 14 in painting, first figure has been shown.The time window of drawing is striden 6 OFDM symbols (wherein preceding two OFDM symbols indicate vertical line).Second figure has shown the digital filter output when filter memory was removed before each new OFDM symbol.Can see that the DC in each OFDM symbol is about constant, and reduce step by step to the next one from an OFDM symbol.Finally, it can perhaps converge to the level (constant portion) of residual DC in the branch near zero.

In emulation, High Pass Filter Cutoff Frequency is set as 2kHz.When using equalization digital filter, the cut-off frequency of the HPF among the RF can be made as 5kHz even higher, and can not damage the recovery of data symbol.High cutoff can cause a rapid adjustment filter, and therefore after an OFDM symbol, the constant DC level in the signal will almost completely be removed in equalization filter.

Some execution mode of the present invention can have one or more following benefits:

● balanced iir filter can be guaranteed to be offset by group delay distortion and frequency distortion that the HP filter in the analog RF is introduced, eliminates the detection performance reduction near the OFDM symbol of DC carrier wave thus.

● can reduce influence from any DC power that generates among the RF (for example, improve oneself resistance).Appear at the middle strong resistance of OFDM symbol and may cause that DC jumps, and reduce the detection of this symbol.Yet, can protect successive character by using equalization filter.When resistance disappeared, similarly DC jumped and will make this only will disturb single OFDM symbol.

In the RF receiver, received in this context of OFDM symbol embodiments of the present invention have been discussed.Yet embodiments of the present invention are equally applicable to other wireless standards, and can use in expectation reduces any situation of the DC power level in the signal be applied to analog to digital converter.

Usually, various execution mode of the present invention can be realized by hardware or special circuit, software, its any combination of logic OR.For example, some aspect can realize by hardware, and other aspects can the invention is not restricted to this certainly by being realized by firmware or software that controller, microprocessor or other computing equipments are carried out.According to an embodiment of the invention, receiver can be implemented as the part of integrated circuit.Although various aspects of the present invention can illustrate and be described as block diagram, flow chart or use other figure to paint expression, but fine understanding, as non-limiting example, these frames described here, device, system, technology or method can be by hardware, software, firmware, special circuit or logic, common hardware or controller or other computing equipments or its some make up and realize.

Above describe to provide the complete and informedness of illustrative embodiments of the present invention is described by exemplary and mode non-limiting example.Yet when reading is above described with claims in conjunction with the accompanying drawings, various modifications and adjustment will become for various equivalent modifications and easily see.Yet all that the present invention is instructed will drop in the scope of the present invention of appended claims qualification with similar modification.

Claims

1. device comprises:

Analog filter;

Be coupled to the analog to digital converter of described analog filter; And

Be coupled to the digital filter of described analog to digital converter;

Wherein dispose described device, make the distortion of introducing through filtering signal by described analog filter compensate by described digital filter in fact.

2. device as claimed in claim 1, one frequency response comes down in described analog filter and the described digital filter the contrary of another in wherein said analog filter and the described digital filter.

3. device as claimed in claim 1 or 2, wherein said analog filter comprises high pass filter.

4. device as claimed in claim 3, wherein said high pass filter have the frequency response of being determined by following formula:

A (s) = \frac{s}{s + \frac{1}{τ}}

Wherein τ is the time constant that is associated with described analog filter.

5. as arbitrary at the described device of preceding claim, wherein said digital filter comprises IIR filter.

6. device as claimed in claim 5, wherein said digital filter have the frequency response of being determined by following formula:

D (z) = \frac{1 - a \cdot z^{- 1}}{1 - z^{- 1}}

Wherein a is a filter coefficient.

7. device as claimed in claim 6, wherein said filter coefficient is determined by following formula:

a = e^{\frac{- 1}{τ \cdot fs}}

Wherein fs is a sample frequency.

8. as each described device of claim 5 to 7, wherein said digital filter comprises filter memory.

9. device as claimed in claim 8, wherein said filter memory are configured to regularly be reset.

10. install as claimed in claim 8 or 9, wherein said filter memory is configured to be reset during the protection period before the reception data symbol.

11., also comprise: be coupling in the low pass filter between described analog filter and the described analog to digital converter as arbitrary at the described device of preceding claim.

12. device as claimed in claim 11 also comprises: be coupling in the baseband amplifier between described low pass filter and the described analog to digital converter.

13., also comprise: be coupling in the channel model between described analog to digital converter and the described digital filter as arbitrary at the described device of preceding claim.

14. as arbitrary at the described device of preceding claim, also comprise: be coupled to the frequency mixer of the input of described analog filter, wherein said mixer configuration is used for the signal and the mixing frequency that receive are carried out mixing.

15. as arbitrary at the described device of preceding claim, wherein said device further configuration is used for received RF signal.

16. device as claimed in claim 15, wherein radiofrequency signal comprises the signal of orthogonal frequency division modulated.

17. a method comprises:

Utilize first filter that analog signal is carried out filtering;

In analog to digital converter to changing through filtered analog signals; And

Utilize second filter that filtering is carried out in described sampling;

Wherein introducing described distortion through filtered analog signals by described first filter is compensated by described second filter in fact.

18. method as claimed in claim 17, one frequency response comes down in described first filter and described second filter the contrary of another in wherein said first filter and described second filter.

19. as claim 17 or 18 described methods, wherein said first filter comprises high pass filter.

20. method as claimed in claim 19, wherein said high pass filter have the frequency response of being determined by following formula:

A (s) = \frac{s}{s + \frac{1}{τ}}

Wherein τ is the time constant that is associated with analog filter.

21. as each described method of claim 17 to 20, wherein said second filter comprises IIR filter.

22. method as claimed in claim 21, wherein said second filter have the frequency response of being determined by following formula:

D (z) = \frac{1 - a \cdot z^{- 1}}{1 - z^{- 1}}

Wherein a is a filter coefficient.

23. method as claimed in claim 22, wherein said filter coefficient is determined by following formula:

a = e^{\frac{- 1}{τ \cdot fs}}

Wherein fs is a sample frequency.

24. as each described method of claim 21 to 23, wherein said second filter comprises filter memory.

25. method as claimed in claim 24 also comprises: the described filter memory of regularly resetting.

26., also comprise: the described filter memory of resetting during the protection period before receiving data symbol as claim 24 or 25 described methods.

27., also comprise: before described conversion, described analog signal is carried out low-pass filtering as arbitrary in the described method of preceding claim.

28., also comprise: the signal and the mixing frequency that receive are carried out mixing, to generate described analog signal as arbitrary in the described method of preceding claim.

29. method as claimed in claim 28, the signal of wherein said reception is a radiofrequency signal.

30. a computer program code means, when described program was moved on processor, described computer program code means was suitable for carrying out as the described arbitrary step of claim 17 to 29.